1. Field of the Invention
The present invention relates to a pulse combustion apparatus for use as a heat source in a water heater.
2. Description of the Related Art
A pulse combustion apparatus is advantageous over ordinary combustion apparatuses in two respects. First, it has high thermal efficiency; second, its exhaust gas has low toxicity.
The conventional pulse combustion apparatus comprises a combustion chamber having an exhaust port, a fuel-supplying system connected to the combustion chamber for supplying fuel gas thereinto, an air-supplying system connected to the combustion chamber for supplying air thereinto, a tail pipe connected to the exhaust port of the combustion chamber, two flapper valves incorporated respectively in the fuel-supplying system and the air-supplying system, and an ignitor incorporated in the combustion chamber for igniting a mixture gas in the combustion chamber, thereby starting combustion therein.
When ignited, the mixture gas in the combustion chamber explodes, raising the pressure in the chamber and automatically closing the flapper valves. At the same time, the combustion gas flows at high speed from the chamber through the exhaust port. The gas is then exhausted into the atmosphere through the tail pipe. As a result of this, the pressure in the combustion chamber decreases to a negative value, causing the flapper valves open by themselves, allowing the fuel gas and air to flow into the combustion chamber from the fuel-supplying system and the air-supplying system. When a specific amount of fuel gas and a specific amount of air have been supplied into the chamber and mixed together, the resultant gas mixture ignites and explodes by virtue of residual flames.
Thereafter, the fuel-air mixture is repeatedly combusted as long as the fuel-supplying system and the air-supplying system keep supplying fuel gas and air to the combustion chamber. The combustion gas in the tail pipe makes an oscillating flow having a large amplitude. Hence, more heat is transmitted from the tail pipe than in the case where the gas makes a steady flow. In addition, since the fuel is intermittently combusted in the combustion chamber, the amount of nitrogen oxides formed is small. The heat generated by the repeated combustions is collected from the outer periphery of the combustion chamber and also from the outer periphery of the tail pipe.
In most pulse combustion apparatuses, the tail pipe has a cross section area smaller than that of the combustion chamber. In other words, the inner diameter of the tail pipe is much smaller than that of the combustion chamber. This is because the tail pipe and the combustion chamber constitute a Helmholtz resonator. The resonator can be regarded, theoretically at least, as comprised of a mass and a spring. The mass is the gas in the tail pipe, and the spring is the gas in the combustion chamber. Further, the source of resonance is the intermittent explosion occurring in the combustion chamber. The resonant frequency f of this pulse combustion apparatus is given: ##EQU1## where a is the mean velocity (m/s) of sound in the tail pipe, s is the cross section area (m.sup.2) of the tail pipe, V is the volume (m.sup.2) of the combustion chamber, and l is the length (m) of the tail pipe.
The pulse combustion apparatus, described above, is disadvantageous in the following respects.
First, since the tail pipe has a small cross section area and, thus, a small inner surface, heat cannot be transmitted through the wall of the pipe with high efficiency. (In other words, the amount of heat transmitted through the wall of the tail pipe is small.) To increase the heat-transmitting efficiency, a secondary heat exchanger is located downstream of the tail pipe and collects the heat which cannot be collected through the wall of the tail pipe. The use of the secondary heat exchanger, however, makes the apparatus not only large but also expensive.
Secondly, the tail pipe, which has an inner diameter far smaller than that of the combustion chamber, is connected to the combustion chamber by means of a conical pipe so that the combustion gas can smoothly flow from the chamber to the tail pipe. The tail pipe, the chamber, and the conical pipe are integrated, forming a one-piece component. In most cases, this component is made by stamping an aluminum plate by using two stamping dies, one for forming the tail pipe and the other for forming the combination of the chamber and the conical pipe. Either stamping die is expensive, and stamping is a time-consuming process. It is inevitably expensive to manufacture said one-piece component.